The present invention is related to conjugate spunbond fibers containing a high melt flow rate propylene polymer and to nonwoven webs produced therefrom.
Spunbond fibers are small diameter filaments or fibers that are formed by extruding or melt-spinning thermoplastic polymers as filaments from a plurality of capillaries of a spinneret. Unlike typical textile yarn and staple fiber production processes which mechanically draw spun filaments, in a spunbond fiber production process, extruded filaments are rapidly drawn while being cooled by a flow of pressurized air or by one of other well-known pneumatic drawing processes. The drawn filaments are deposited or laid onto a forming surface in a random, isotropic manner to form a loosely entangled fiber web, and then the laid fiber web is bonded to impart physical integrity and dimensional stability. The production of spunbond webs is disclosed, for example, in U.S. Pat. Nos. 4,340,563 to Appel et al.; 3,692,618 to Dorschner et al. and 3,802,817 to Matsuki et al. Spunbond fibers have relatively high molecular orientation, compared to other fibers produced with a pneumatic drawing process, e.g., meltblown fibers, and thus exhibit relatively high strength properties.
Conjugate fibers having two or more component polymers that are designed to benefit from combinations of desired chemical and/or physical properties of the component polymers are well known in the art. Methods for making conjugate fibers and fabrics produced therefrom are disclosed, for example, in U.S. Pat. Nos. 3,595,731 to Davies et al., Reissue 30,955 to Stanistreet and 5,418,045 to Pike et al., and European Patent Application 0 586 924. It is also known that nonwoven webs containing crimped conjugate fibers exhibit improved tactile properties, including bulk, softness and fullness. For example, U.S. Pat. No. 5,418,045 discloses a nonwoven fabric of crimped conjugate spunbond fibers that has highly desirable textural properties and improved fiber coverage. The patent teaches a spunbond nonwoven fabric production process that draws and thermally crimps conjugate spunbond fibers before the fibers are deposited to form a nonwoven fabric.
Although processes for thermally crimping conjugate fibers are known in the art, the process of thermally imparting crimps during the production process of the fibers becomes highly onerous as the average size (thickness) of fibers is reduced to produce fine denier fibers and/or the throughput, i.e., the amount of polymer processed through the spinneret, of component polymers for the conjugate fibers is increased to speed up the production. Consequently, attempts to produce small denier fibers and to increase the throughput or production rate tend to result in flat and dense nonwoven webs. This difficulty in imparting crimps is especially pronounced in the production of spunbond fibers since the pneumatic drawing step of a spunbond fiber production process, unlike a mechanical draw process, provides only a limited drawing force and does not draw the spun fibers with the high drawing ratio capabilities of a mechanical drawing process.
There remains a need for a process for producing highly crimped pneumatically drawn conjugate fibers that can impart high levels of crimps even for fine denier fibers and even at high speed production rates without requiring additional and onerous manufacturing steps.